Polyphenol is a generic term of compounds having two or more phenolic hydroxyl groups in the same molecules. The number thereof amounts to 5000 or more and it is classified into phenylcarboxylic acid-based, lignan-based, curcumin-based, coumarin-based, or flavonoid-based one. Polyphenol is a varied sugar which is changed in part, is contained in leaves, flowers, bark, and the like of plants, is synthesized by photosynthesis, and has an action of deactivating active oxygen generated by ultraviolet rays.
Lifestyle-related diseases such as cancers and cardiac diseases are developed through DNA damage, breakage of cell membranes, protein denaturalization, receptor injury and the like caused by active oxygen generated in fat-soluble components between cells and on cell membranes. Polyphenol may induce apoptosis of cancer cells and also deactivate active oxygen through the antioxidant action, so that polyphenol is said to be effective for prevention of these diseases (Non-Patent Documents 1 and 2).
Therefore, polyphenol attracts attention as an active ingredient of functional foods and studies on the search for plants containing polyphenol have been actively performed. However, an attempt to increase the content of polyphenol in a plant is hardly known and a method of increasing the content of flavonoid, which is one kind of polyphenol, using a specific acylcyclohexanedione is only disclosed in Patent Document 1 recently laid-open to public.
A series of 5-aminolevulinic acids such as 5-aminolevulinic acid represented by NH2CH2CO(CH2)2COOH, derivatives of its alkyl esters, etc., hydrochlorides thereof, and the like are known to be useful as a photosensitizer in photokinetic therapy (Patent Document 2), a plant growth regulator (Patent Document 3), a herbicide (Patent Document 4), a therapy of infection with fish pathogenic microorganisms and parasites (Patent Document 5), a pig growth accelerator (Patent Document 6), a plant oligosaccharide-increasing agent (Patent Document 7), and the like. However, a polyphenol-increasing effect thereof is entirely not known.    Patent Document 1: JP-A-2006-52217    Patent Document 2: JP-T-2004-505105    Patent Document 3: JP-A-07-53487    Patent Document 4: JP-A-05-117110    Patent Document 5: JP-A-2001-316255    Patent Document 6: JP-A-2003-40770    Patent Document 7: JP-A-09-87108    Non-Patent Document 1: Br J Pharmacol 123:565-573, (1998)    Non-Patent Document 2: Oncogene 24: 7180-7189, 2005